1. Field of the Invention
This invention relates to an apparatus for automatically controlling the white-balance of a color video camera.
2. Description of the Related Art
A color imaging system typically comprises a white-balance controlling apparatus for displaying an object on the screen in colors as close as possible to those viewed through the human eye.
A human eye identifies a white color object as it is when the color temperature of light illuminating the object is within a predetermined range, irrespective of the fluctuation. The color of an object captured by the color imaging system, however, originally tends to blue proportionally to the increase of the light color temperature, while it tends to redden proportionally to its decrease. This means that the colors recognized by the color imaging system and by the human eye differ mutually depending on the color temperature of the illumination light.
A white-balance controlling apparatus has been used to adjust such a difference and to give correct levels of both colors. This apparatus acts to correct color data obtained by imaging an object in accordance with the color temperature of an illumination light.
FIG. 24 of the accompanying drawings shows an example of an imaging apparatus such as color video cameras, etc. comprising a predecessor automatic white-balance controlling apparatus, which is a lightly-modified apparatus based on the technical concept disclosed in Japanese Patent Publication No. 43872/1979.
in FIG. 24, the apparatus includes a lens 1, an imaging element 2, and a color separating circuit 3. The lens 1 receives an optical image in a imaging apparatus such as color video camera etc.. The color separating circuit 3 separates the signal obtained by photoelectrical conversion into three color signals of R, G, and B.
These three color signals are supplied to the processing circuit 6 in the following manner respectively: the signal of R (hereinafter called tile R signal, and similarly for the other two signals), through a gain control circuit (hereinafter called as GC); the G signal, directly; the B signal, through GC5. The processing circuit 6 generates a brightness signal (hereinafter referred to as Y signal) and a color difference signal from the R signal, G signal, and B signal. There are two types of color difference signal. These are, R-Y signal representing the level difference between the R signal and the Y signal, and B-Y signal representing the level difference between the B signal and the Y signal. These Y, R-Y, and B-Y signals are encoded by an encoder 7 and output therefrom as a color video signal.
Aforementioned are a composition of signals generally not relevant to the white-balance control, which is performed by adjusting the levels of the R signal and the B signal by GC4 and GC5. GC4 and GC5 are circuits which adjust the level of the R signal or the B signal with a gain variable in accordance with the gain control signal. The gain control signal is generated by the following composition. Namely, the composition hereinafter described performs negative return control for the GC4 and the GC5 on the basis of the detected result of the integrated value of the R-Y signal and the B-Y signal, and is called an automatic white-balance controlling apparatus.
FIG. 24 shows integrator circuits 8 and 9. The integrator circuit 8 integrates R-Y signal while the integrator circuit 9 integrates B-Y signal. The integrated values of the R-Y signal and the B-Y signal obtained in the integrated circuits 8 and 9 respectively are supplied to the non-inverse input terminal of the comparators 10 or 11. The inverse input terminals of the comparators 10 and 11 receives a reference voltage Vref from a reference voltage generator 15. The comparator 10 and 11 compare the integrated values of the R-Y signal and the B-Y signal to the reference voltage Vref and output a signal representing the compared result (hereinafter, the signals output from the comparators 10 and 11 are called RchCOMP and BchCOMP respectively).
RchCOMP and BchCOMP are supplied to the microcomputer 12, which generates Rch0UT and Bch0UT based on RchCOMP and BchCOMP and outputs them in accordance with the clock pulse. Rch0UT and Bch0UT correspond to either the R signal or the B signal of the gain control signal, and are generated in accordance with a predetermined algorithm. The microcomputer 12 outputs one coupled is a D/A converters 13 and 14, where the former and the latter convert respectively Rch0UT and Bch0UT into analog gain controlling signal. A gain controlling signal for Rch0UT is supplied to GC4 for controlling the R signal level, while a gain controlling signal for Bch0UT is supplied to GC5 for controlling the B signal level. However, GC4 and GC5 increasingly changes the gains for the values of the gain controlling signals linearly.
This control is performed such that the R-Y signal and the B-Y signal become zero when an achromatic color is imaged. The reason why such a control is possible is that; when a scene is imaged, if the R-Y signal in respect of the entire scene is integrated, the conditions become the same as in the case when an achromatic color is imaged, and if the B-Y signal in respect of the entire scene is integrated, the conditions again become the same as in the case when an achromatic color is imaged. Such adjustment would assure a certain white balance irrespective of the change of the light source for illuminating the object.
Comprising such a composition, the operation of an automatic white-balance controlling apparatus has been dependent on the operation of the microcomputer 12. FIG. 25 shows an example of the operation of the microcomputer 12.
As shown in FIG. 25, immediately after the starting of the operation, the initializations of Rch0UT, BchOUT, RchUDC, BchUDC are carried out (S1). These RchUDC and BchUDC are original values for RchOUT and Bch0UT respectively, and can be obtained by the following reversible calculation. Upon completion of the step S1, a calculation loop is entered as shown in the drawing.
In this loop, firstly it is judged whether the H value or the L value should be selected from the comparing circuit 10 (S2). The case where RchCOMP from the comparing circuit 10 is the H value represents that the integrated value of R-Y signal is above the reference value Vref. The reference value Vref is set in accordance with the value of the color difference signal (near the 0V). Therefore, the case where RchCOMP from the comparing circuit 10 is the H value results from the average value of the R-Y signal for one screen being higher than the average value of R-Y signal in the case where an achromatic (white) color object is imaged. Meanwhile, on the contrary, the case where RchCOMP from the comparing circuit 10 is the L value results from the average value of the R-Y signal for one screen being lower than an average value of R-Y signal in the case where an achromatic (white) color object is imaged. After step S2, a decrement of RchUDC is executed when it is H value (S3), while RchUDC increment is executed when it is the L value (S4).
Next, it is judged whether the H or L value is selected from the comparing circuit 11 (S5). In the same manner as step S2, the case where BchCOMP from the comparing circuit 11 is the H value results from the average voltage of B-Y signal for one screen being higher than the average voltage of B-Y signal of the case where an achromatic (white) object is imaged. On the contrary, the case where BchCOMP from the comparing circuit 11 is the L value results from the average voltage of B-Y signal for one screen being lower than the average voltage of B-Y signal of the case where an achromatic (white) object is imaged. In step S5, an increment of BchUDC increment is executed when it is judged as the H value (S6), and a decrement of BchUDC is executed when it is judged as the L value (S7).
Thereafter, RchOUT and Bch0UT are output (S8). RchOUT and BchOUT are generated based on the values of RchUDC and BchUDC. Most simply, RchUDC and BchUDC themselves are used as they are. After step S8, clock pulse-input waiting is executed (S9), and the loop returns to step S2 upon receiving the input.
Thus, RchUDC and BchUDC are obtained from the information representing the level of the average voltage for one screen and on the basis of the comparison with the reference voltage Vref. Based on these values for RchUDC and BchUDC, the level adjustment for the R signal and B signal carried out. As a result, it is possible to exercise control such that R-Y signal and B-Y signal are zero when an achromatic object is imaged as mentioned above.
FIG. 26 shows another example of the operation of the microcomputer 12 where RchUDC and BchUDC are dealt as RchOUT and BchOUT respectively. Accordingly, unlike the case shown in FIG. 25, no initialization of RchOUT and Bch0UT is performed in step S1', while RchUDC and BchUDC are output in step S8' after step S9.
FIG. 27 shows another example of an imaging apparatus for a color video camera etc.. This apparatus also comprises a similar conventional automatic white-balance controlling apparatus but different in having a automatic gain controller 116 between the imaging element 102 and the color separating circuit 103. In this composition, it is possible to stabilize the white-balance adjustment by controlling the input level of the color separating circuit 103.
However, several disadvantages have arisen in such conventional automatic white-balance controlling apparatus.
Firstly, when green colored objects occupy a majority of the visual imaging field and therefore green is the major color in the screen, the GC for the R signal and the GC for the B signal cause the GC circuits to enter an excessively gained state. In consequence, the image of a white object, which should be seen in white color in the screen, would be colored in magenta. Namely, the white-balance breakes, thereby for example the blue sky would appear inclined to white, and the skin of a Japanese would appear white.
Secondly, the output value of the comparing circuit would be unstable, if noise were introduced to its input. Namely, the white-balance oscillates.
Thirdly, the white-balance tends to be unstable when there is too short a distance to the object. When the distance to the object is long, the objects can usually be within the visual field. Therefore, the integration of the color difference signal can be executed based on the many objects, providing stable white-balance control. However, when the distance to the object is short, the number of objects that fall within the visual field becomes too small, namely only one or the like, thereby the polarity of the output of the comparing circuit tends to reverse. As a result, the white-balance control tends to be unstable.
Fourthly, with the use of a zooming lens if the zooming ratio is on the tele-side, the white-balance control would tend to be unstable. This is because; when the zooming ratio is on the wide side and a number of objects can be within the visual field, the integration of the color difference signal can be executed on the basis of the various objects. However, in this case the number of objects to be within the visual field is small, one or the like, thereby the output of the comparing circuit can reverse frequently.
Further, the white-balance would tend to be unstable when the lightness of the object is insufficient. Namely, if the object has sufficient lightness and the level of the signal from the imaging element is large enough to ignore the noise, the white-balance controlling operation would stabilize. But it the lightness is insufficient, the output polarity of the comparing circuit would tend to reverse due to the noise.